1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Generally, an electrophotographic image forming apparatus (such as a printer, copier, and facsimile machine) applies laser light based on image data to a charged photoconductor (light exposure) to form an electrostatic latent image. Then, the image forming apparatus directly or indirectly transfers a toner image, which is formed by causing toner to adhere to the electrostatic latent image, onto a sheet, and then heats and presses the toner image to fix the image to the sheet, thereby forming an image on the sheet.
A fixing device of the image forming apparatus that performs the above-mentioned fixing is occasionally provided with an air separator that applies air toward an end portion of a sheet having passed through a nip portion, in order to prevent a fixing defect caused by a sheet wound around a heating section for heating a toner image (for example a fixing belt). In particular, since a thin sheet with a small basis weight tends to easily wind around the heating section, it is required to direct high-speed air jets toward such a sheet. In order to deal with such a requirement, the air separator typically has a duct having a form in which its air outlet is substantially smaller than its air inlet.
There are known compressor type air separators that use a compressor as a source of air, and fan type air separators that use a fan motor as a source of air (for example, see Japanese Patent Application Laid-Open Nos. 10-265067 and 11-157678).
When the compressor type air separators and the fan type air separators are compared, the fan type air separators have advantages over the compressor type air separators in that the size can be reduced, manufacturing cost can be kept low, and the operation noise is small. However, since fan type air separators have significantly low static pressure as compared to compressor type air separators, a significantly large air quantity is required for obtaining an air velocity at which the sheet separation can be caused, and heat loss at a heating section is accordingly great. In addition, when the air quantity is large, a sheet having passed through a nip portion may flap, and as a result, wrinkles may be formed on the sheet and a sheet ejection path may be jammed with sheets, making ejection of sheets impossible. Therefore, it is important not only to ensure an air quantity required for obtaining an air velocity at which the sheet separation can be achieved, but also to suppress the air quantity to the degree that side effects such as the above-mentioned heat loss and flapping of sheets are not caused.
For example, Japanese Patent Application Laid-Open No. 2008-197654 discloses a technology for optimally adjusting the quantity of air from an air source. A fixing device disclosed in Japanese Patent Application Laid-Open No. 2008-197654 measures the amount of air pressure in an air passage by using a pressure sensor and optimizes the quantity of air by a feedback control based on a measured pressure value.
However, in existing fan type air separators, pressure loss of air passing through a duct is caused due to a substantially narrowed air outlet. When the feedback control of air based on the measured pressure value is performed in such a state, it is difficult to stabilize the air velocity since responsiveness to control is not good. As described, when the velocity of air jets directed toward sheets passing through a nip portion is unstable and varies, the position of sheets separated from the heating section may become unstable, and thus a jam may be caused in the sheet ejection path.